Effective Layout Design for Laser Fault Sensor on FPGA

Laser fault injection (LFI) refers to a precise attack that introduces specific errors into an operating device. In response to the increasing prevalence of such attacks, recent studies have proposed various countermeasures, primarily including conventional analog circuit-level solutions such as optical sensors and current sensors. In addition, digital sensors can be designed at the digital circuit level. However, the countermeasures proposed using digital sensors have primarily been limited to the schematic level, and their physical layouts have not been thoroughly examined. To this end, this study proposes a novel design methodology focusing on the physical layout of digital sensors to enhance laser detection in field programmable gate arrays. The proposed design methodology was applied to two types of sensors, namely, ring oscillator (RO)-based and time-to-digital converter (TDC)-based sensors. First, we conducted a naive implementation of the RO-based sensor, which did not consider the layout and only provided protection to a limited area of the device. Moreover, we proposed a more detailed design methodology for digital LFI sensors, which was specifically tailored to effectively protect a larger area of the device and was successfully applied to both RO-based and TDC-based sensors. In addition to this design methodology, we conducted comprehensive laser-scanning experiments using an extensive parameter space to evaluate the two types of LFI sensors. These evaluations demonstrated that the improved RO-based sensor can detect up to 80.1% of laser shots and 99.8% of the faults. In comparison, the TDC-based sensor could detect 75.4% of the laser shots and 85.4% of the faults. These result shows the effectiveness of the proposed method.